1. Field of the Invention
The present invention relates to a direct-injection diesel engine in which a shallow-dish type piston cavity is formed on the top surface of a piston, and fuel is injected from a fuel injector and burned in a fuel-air mixture state. More specifically, the present invention relates to a direct-injection diesel engine in which the distance between opposite wall portions of the shallow-dish type piston cavity is set to a predetermined value; a squish area defined by the top surface of the piston and a cylinder head forms a part of the combustion chamber as a whole; and in the vicinity of the top dead center, the velocity of a reverse squish flow from the shallow-dish type piston cavity to the squish area, generated by a movement of the piston and represented by Vs, and the fuel spray velocity in the vicinity of a lip portion of an open wall portion of the shallow-dish type piston cavity, represented by Vsp, are set such that the ratio Vs/Vsp is not greater than 1.25, so as to attain proper balance between the fuel-spray and the velocity of the reverse squish flow in order to cause the fuel to uniformly disperse within the shallow-dish type piston cavity and the squish area and to burn uniformly, to thereby reduce exhaust emission of toxic substances. The present invention also relates to a combustion method for the diesel engine.
2. Description of the Related Art
In a conventional diesel engine, combustion is mostly performed within a piston cavity and combustion in a squish area is avoided.
Recently, society has demanded the reduction of toxic exhaust emissions from small direct-injection diesel engines for automobiles. An effective measure for meeting such demand is to decrease the diameter of the nozzle hole of a fuel injector to thereby promote atomization of injected fuel. Therefore, the nozzle hole diameter has tended to decrease.
Meanwhile, from the viewpoint of reducing emission of CO.sub.2, which contributes to the greenhouse effect, output power per unit displacement (specific output) is preferably increased as compared to the case of conventional engines in order to enable a smaller engine to output power comparable to that of a relatively larger conventional engine, which leads to the benefit that such a smaller engine decreases the weight of a vehicle and improves fuel efficiency. For this purpose, a turbocharger or the like is desirably employed for performing supercharging, to thereby force a greater amount of air into a cylinder, and the engine is desirably operated at high speed, as is the case with a gasoline engine.
That is, in order to reduce emission of toxic substances, demands have been placed on conventional diesel engines to realize a reduced nozzle hole diameter, an increased degree of supercharging, and an increased engine speed.
In a large-sized diesel engine for marine use or stationary use, since the engine speed is low, there is provided sufficient time for achieving complete combustion. In addition, the distance from an injection nozzle to a wall surface is long, and therefore fuel spray combusts without impinging on the wall surface. Therefore, the characteristics of combustion essentially differ from those found in a small-sized diesel engine for automobiles.
The conventional concept employed in the design of a small-sized direct-injection diesel engine for passengercars and commercial vehicles is as follows: within a period when a piston remains in the vicinity of top dead center or when the ratio of the volume of a squish area to the volume of the whole combustion chamber is small, combustion of fuel is completed within the piston cavity while the fuel is prevented from flowing out into the squish area. The diameter of the combustion chamber suitable for such combustion has been set to a relatively small value, i.e., the ratio of the outermost diameter of the piston cavity to the piston diameter has been set to 0.52 or less.
When the above-described conventional diesel engine is designed to achieve a reduced nozzle hole diameter, an increased degree of supercharging, and an increased engine speed, which will be demanded in the future, fuel cannot be burned uniformly within the piston cavity. That is, even though the velocity of fuel spray decreases due to the reduced nozzle hole diameter and the increased degree of supercharging, a reverse squish flow velocity increases due to the increased operation speed, so that the fuel spray velocity and the reverse squish flow velocity fall out of balance. Consequently, fuel vapor stagnates locally in a region extending from the upper portion of the combustion chamber to the squish area, and a nonuniform spatial distribution of fuel is thus produced, resulting in increased emission of toxic substances.
That is, among the above-described reduced nozzle hole diameter, increased degree of supercharging, and increased engine speed, the reduced nozzle hole diameter and the increased degree of supercharging (i.e., increased air density at the time of fuel injection) decrease the fuel spray velocity, whereas the increased engine speed increases the velocity of air flow; i.e., the reverse squish flow velocity. As a result, in a combustion chamber according to the conventional combustion concept, the balance between the fuel spray velocity and the air velocity is upset, so that the fuel spray velocity becomes excessively low in relation to the air flow velocity, and fuel is present locally in the region extending from the upper portion of the piston cavity to the squish area. In this case, air in the lower portion of the piston cavity cannot be used. This uneven spatial distribution of fuel prevents reduction in emission of toxic substances.
Further, the three above-described trends themselves have caused conflict with the conventional combustion concept. That is, the decreased nozzle-hole diameter decreases injection quantity per unit time, so that the injection duration tends to increase. Also, the increased degree of supercharging increases the fuel injection quantity by an amount corresponding to an increase in air intake caused by the supercharging, so that the injection duration increases. Moreover, the increased engine speed results in an increased crank angle per unit time, so that the crank-angle based fuel-injection duration increases. When an attempt is made to simultaneously satisfy the above-described requirements, the crank-angle based fuel-injection duration becomes as large as 40.degree..
If the fuel injection period becomes as long as above, the piston has moved to a considerably downward position when fuel injection is completed. In this case, it becomes impossible to conform to the conventional concept that combustion of fuel is completed within a piston cavity while the fuel is prevented from flowing out into the squish area. Accordingly, in the conventional concept, a combustion chamber that satisfies the three above-described requirements cannot be realized.